Most vehicle suspension systems utilize damping devices or shock absorbers for controlling the vibrations of the body and wheel due to road disturbances imposed on the mass-spring system of the vehicle body/wheel and suspension springs. A vehicle suspension damper usually provides a resistive force proportional to the relative velocity between the body and the wheel. High performance controlled damping applications, such as those used in passenger vehicle suspension systems, preferably provide a relatively low damping force at low speeds for comfort, and provide a relatively high damping force at high speeds for safe handling of the vehicle. It is known that such response characteristics can be provided by semi-active or active suspension systems, wherein the damping response of the systems can be continuously varied in real time in response to the dynamic conditions experienced by the vehicle using continuously variable—real-time damping (CV-RTD) actuators.
The use of CV-RTD actuators using “smart fluids” (e.g., electrorheological (ER) and magnetorheological (MR) fluids) with continuously variable and controllable rheology and a fixed flow portion instead of moving mechanical valves with a variable flow portion have been proposed. The use of ER fluids requires relatively large electrical fields (on the order of 5 kV/mm to produce the desired range of rheological effects, whereas MR fluids produce similar Theological effects at voltages well below 12V and hence have generally been preferred for many applications, including use in automotive vehicles.
Magnetorheological (MR) fluids consist of magnetizable particles (e.g., iron and/or iron alloy powders) suspended in an inert base fluid (e.g., synthetic oil). MR fluids typically exhibit Newtonian flow characteristics, with negligible yield stress when there is no external magnetic field. However, the yield stress of a MR fluid can be increased by several orders of magnitude by subjecting it to a magnetic field perpendicular to the flow direction of the fluid. This Bingham plastic behavior of MR fluid in the “on” state is advantageous in creating actuators with controllable force or torque characteristics such as vibration dampers and clutches, without using any moving valves. MR fluids, and devices using the MR fluids, are well known. However, earlier problems with sedimentation and abrasion discouraged their use. Recent advances in material technology and electronics have renewed the interest in MR fluids for applications in smart actuators for fast and efficient control of force or torque (e.g., damping) in a mechanical system.
Other examples of CV-RTD dampers are described and illustrated in U.S. Pat. Nos. 5,277,281 and 6,390,252 and generally comprise monotube MR dampers 10 having a piston 20 sliding within a hollow tube 30 that is filled with MR fluid 40, as illustrated in FIG. 1.
While MR fluid dampers of the types generally described above have been successfully used in CV-RTD applications and have demonstrated the ability to provide fast and continuous variable control of damping forces, they require that the hollow tube be substantially filled with MR fluid which is a relatively expensive material. Typically vehicles equipped with MR fluid shock absorbers use on the order of 1 liter or more per vehicle. Furthermore, the fluids may require special handling and disposal at the end of the useful service life of the vehicle. These devices typically require a special finish on the rod and the inner surface of the tube, and special high pressure seals for the floating gas piston and the rod to minimize abrasion associated with the MR fluid and provide the necessary sealing at the operating fluid pressures of the device. Also, the packaging of the piston with an appropriate coil can be difficult when high turn-up ratios at high velocities are desired due to the relatively large size of the coil required to provide these ratios.
It is therefore desirable to develop damper designs that utilize MR fluid to control damping forces, but which also eliminate or reduce some of the requirements associated with the prior damper designs, such as the volume of MR fluid required, the need for special component finishes, the need for a gas reservoir to accommodate the MR fluid displaced during the actuation of the damper, or the need for coil designs that are difficult to package in the envelope allowed for the dampers.